Hexakis (alkylisonitrile) technetium(I) complexes are a new class of lipophilic cationic myocardial perfusion imaging agents that have the potential to replace thallous chloride (201T1) for the clinical evaluation of coronary artery disease. Two complexes have recently emerged from clinical trials showing significant promise for t\routine clinical use, hexakis (carbomethoxyisopropyl isonitrile) 99mTc (Tc-CPI) and hexakis (methoxyisobutyl isonitrile) 99mTc (Tc-MIBI)> Although empirically developed and already utilized in clinical trials, little is understood about the mechanisms of myocardial uptake and retention of these agents. The experiments proposed in this application will undertake and retention of this important new class of compounds. By analogy to other lipophilic cations that distribute across biological membranes according to transmembrane potential, this study will test the hypothesis that tc- isonitrile complexes are probes of mitochondrial and plasma membrane potential. Chick embryo heart cell in monolayer culture in combination with 99mTc and 86Rb isotopic tracer methodologies and cellular fractionation techniques underpinned with membrane biophysical analysis will be used to evaluate the cellular and subcellular mechanisms of Tc-isonitrile myocardial uptake and retention. Correlative ATP and metabolic inhibition will be determined. Biochemical analysis and metabolic inhibitory manipulations in isolated rat mitochondrial and liposome preparations will allow direct quantitative analysis of the distribution of Tc-isonitriles across membranes. Finally, armed with the insight gained from the cellular and subcellular studies, Tc-MIBI will be applied as a probe of mitochondrial membrane potential in vivo in isolated perfused guinea pig heart to noninvasively study the relationship of high energy phosphate metabolism monitored by 31P-NMR spectroscopy and 99mTc tracer analysis during reversible hypoxic injury. These studies could have direct and immediate application to the clinical development of these unique complexes as myocardial perfusion imaging agents as well as their potential application as noninvasive probes of membrane potential in the physiological evaluation of myocardial energetics.